Nonionic Water-Soluble Polysilanes: Global Conformation in Solution and the Occurrence of Preferential Solvation

In aqueous solution UV-absorption and fluorescence emission spectroscopy reveal that the nonionic water-soluble polysilanes poly(4,7,10-trioxaundecylmethylsilane) ( 1 ) and poly(4,7,10,13-tetraoxatetradecylmethylsilane) ( 2 ) are present in a highly folded random coil. In contrast, in organic solvents the backbone adopts a more extended conformation. These changes in backbone conformation are accompanied by changes in the average backbone silicon segmental length L from 15 to 21−24. Concomitantly, the exciton coherence length also exhibits a 2-fold increase. The results obtained for 1 and 2 are compared to those of poly(4,7,10-trioxahexadecylmethylsilane) ( 3 ) and poly(4,7,10,13-tetraoxanonadecylmethylsilane) ( 4 ), which contain instead of a CH 3, a C 6 H 13 moiety as the side chain end group. A detailed analysis of aqueous/organic binary solvent mixtures of 2 reveals that already a low mole fraction of the organic solvent induces large shifts in the photophysical properties; i.e., preferential solvation occurs at a mole fraction of ca. 0.15. Hence upon addition of small amounts of organic solvents, the solute−solvent interactions, which in H 2 O prevent the silicon backbone from adopting a more extended conformation, are reduced, causing the backbone to be less distorted. This is supported by the increase in the quantum yield Φ in going from polar aqueous to apolar organic solvents.